Selection and timing of doses for subjects: The dosage schedule will be flexible with a maximum dose for each subject being 30 mg/kg/day. The following Table provides examples of subjects of different weights receiving increases of dosages. In any individual subject, dose escalation may proceed more slowly, or the dose may be reduced if necessary. At the 4 week visit, if an additional dosage increase is prescribed by Dr Singer, the pharmacy will provide an additional vial of capsules labeled as study drug, but containing either 250 (A) or 500 (B) mg tablets of D-serine or placebo (P); capsule content to be determined by patient's weight (see below). No changes in dosage will be made during the final week of treatment.